Rotating bodies, such as rotors, are used in many different types of mechanical and electrical elements, including generators, motors and other similar devices. These rotating bodies have multiple torsional natural frequency modes and for a variety of reasons, including stress, fatigue, performance, etc., it is desirable to keep these frequency modes within certain ranges. For example, generators, or other mechanical elements including a rotating body, typically have at least one torsional natural frequency mode close to twice a line frequency. If this frequency mode becomes too close to twice a line frequency and becomes excited, it can cause failure of elements in a coupled body, such as the last stage buckets in a coupled turbine.
Currently, the frequency of a rotating body torsional mode can be shifted by changes in either inertia or torsional stiffness that directly impact the frequency of the rotating body mode of interest, i.e., by adding or removing large shrunk-on rings. Therefore, the current method for tuning the frequency of a rotating body torsional mode that is operating at or near a torsional natural frequency requires, minimally, decoupling the rotor from the prime mover and exposing the rotor to allow the installation/removal of enclosure rings over the coupling. These are large, high strength and expensive rings. If this method does not provide a significant enough shift in the torsional natural frequency mode then the rotating body, or components in the coupling, need to be machined to remove stiffness or inertia depending on the scenario. Machining is generally not easily reversible if the modifications do not work because it typically requires an on-site lathe or completely removing the generator from the field and sending it to a service shop for machining. Each of these steps would add significant expense to the solution and could cause an extension in the outage if extensive work was required.
These current methods for tuning the frequency of a rotor torsional mode are further undesirable because they do not target just the torsional frequency, or vibration, of the rotor modes. Instead, the current methods of adding mass, i.e., rings as discussed above, can affect the frequency of other objects connected to the rotating body which can result in unacceptable stress levels or unwanted lateral frequency changes.